What do you mean by ‘infectivity’?

Is the ebolavirus mutating to become more transmissible in humans? Why was Zika virus able to move across the world so rapidly? Why are some viruses able to spread via aerosols floating in the air?

While these may all appear to be very distinct questions, there is one major factor that connects them: the concept of infectivity. Yet it may be a surprise to learn that virologists don’t really understand how ‘infectivity’*** works but Professor Richard Hardy, from Indiana University in the USA, who we talked with for the CVR’s Contagious Thinking podcast, thinks his lab is getting to grips with the causes and consequences of viral infectivity and by harnessing this knowledge he may be able to create more effective and safer vaccines against viruses like chikungunya virus.

***See the bottom of the page for an explainer of what we mean by infectivity.

Richard Hardy

Alphaviruses – a model to understand arbovirus infectivity

Alphaviruses are medically important RNA viruses, which can cause acute febrile illnesses in humans, are spread by mosquitos and are found across the world. While perhaps the most well known is chikungunya virus, a closely related virus, Sindbis virus, is often used as a laboratory model for alphaviruses because it is safer to work with and there are more tools available to study it.

While studying how Sindbis virus infects different mammalian and mosquito cells in the lab, the Hardy lab made the observation that its infectivity was not a static value, rather it was affected by the host cell that the virus was coming from and the host cell in which it was to infect. It even changed over time with the infectivity increasing as the infection progressed. Curiously, viruses coming from mosquito cells had a higher infectivity when added to human cell lines while those coming from human cells were less infectious on either cell line.

When they looked to see why the viruses grown in both cell types had different infectivity they discovered that the human virus was actually a mixture of two kinds of particles with different masses: one ‘heavy’ and another ‘light’ while the mosquito virus population was homogenous and looked more like the heavy kind. It was the light one that was less infectious, replicated slower and induced greater amounts of interferon when added to human cells. Importantly, this wasn’t genetic: if you purified light and infected cells, out came both light and heavy. So what was explaining the increase in mass? And why did that make the virus more infectious?

A rash caused by chikungunya virus infection.

A new model of infectivity

It turns out the difference in mass appears to be explained by the incorporation of components of the mosquito or human host cell small ribosomal subunit and the 5’ ‘capping’ of the incoming viral genomes. Heavy and mosquito-derived particles associated with ribosomal structures and were more likely to be capped; yet one of these features was not necessary for the other to occur. Binding to ribosomal components and capping promotes efficient early translation and likely prevents activation of the innate immune system, which allows the infection to take hold, thus increasing infectivity.

While the molecular details of how the virus genomes bind to host ribosomes and keep them bound and even how this promotes translation are unknown (it’s not thought they are translationally active) it begs the question of why a virus would not make only heavy, more infectious particles, and whether or not there is an evolutionary advantage to making less infectious particles? Indeed, the viral proteins required for capping are known, so why not just cap all your genomes? Perhaps there is some trade-off between capping activity and other viral functions? And could we engineer alphaviruses to make different amounts of non-infectious particles and test these as new vaccines?

This work highlights the complexity in understanding even the most fundamental property of viruses. The events governing infectivity are likely to be different for each kind of virus yet one thing is certain that only by studying these basic biological questions are we able to put ourselves in the position where we may exploit Nature to tackle some of the world’s worst diseases.

Infectivity is central to virology: all viruses must infect a host cell in order to survive. A cycle of infection requires the virus to bind to a cell, enter inside, replicate and then build new virus particles and exit the cell in order to continue to infect more cells, and on and on. This is what occurs within an infected organism, hundreds of thousands of times, and it’s what helps viruses to transmit and ultimately, cause disease. Thus, understanding infectivity is essential to our control of them. But what do we even mean by ‘infectivity’? Are all virus particles infectious?

For a start, there are many different outcomes of an attempted infection:

1) the virus might not be able to gain entry to the cell, so it may never even begin to infect it;

2) the virus can get in but not replicate;

3) the virus enters, replicates and makes new copies of itself but these new viruses are not able to infect new cells.

The ideal situation (and to some extent #3) are classed as productive infections, while # 1 and #2 are considered ‘abortive’. To imagine why this is important, all our efforts to control viral infection relate in some way to manipulating these numbers either by lowering the total infectivity or by reducing the number of infectious units. You could think that even during an infection your body and immune system is trying to maximise the amount of abortive infections compared to the productive ones.

The particle to infectivity ratio

The infectivity of a given stock of virus will be defined by the ability of the virus particles to establish a productive infection in comparison to the total number of virus particles added. This is generally thought of as the particle to infectious unit ratio. On the one hand, we can count virus particles either by electron microscopy or by quantifying the number of genomes and on the other, we can measure the ability of a virus to infect new cells by adding it to cells in a dish, allowing the virus to enter and replicate and then by looking for signs of an infection, such as viral genome amplification, protein production or even generation of new infectious particles. While you can’t have more than one infectious unit per particle, the closer the particle: infectious unit ratio is to 1 then the more infectious your virus is.

For example: if you have 100 virus particles but only 1 (ratio of 100:1) is able to establish a productive infection then that wouldn’t be very infectious. So what is the difference between an infectious and a non-infectious virus particle? That’s where the Hardy lab’s work on a group of viruses called alphaviruses comes into it.